Implement guess key size.

src/set1.rs

@@ -100,7 +100,33 @@ pub fn challenge6() {
         _write("data/6.txt", &bytes_base64);
     }
 
+    fn rate(Bytes(v): &Bytes, size: usize) -> f32 {
+        let (mut rating, mut iterations) = (0, 0);
+        for c in v.chunks(size * 2) {
+            if c.len() < size * 2 {
+                break;
+            }
+            let (a, b) = c.split_at(size);
+            rating += Bytes::hemming(&Bytes(a.to_vec()), &Bytes(b.to_vec()));
+            iterations += 1;
+        }
+        (rating as f32 / iterations as f32) / size as f32
+    }
+
+    fn guess_key_size(bytes: &Bytes) -> usize {
+        let (mut lowest_rating, mut lowest_size) = (f32::MAX, 0);
+        for key_size in 2..40 {
+            let rating = rate(&bytes, key_size);
+            if rating < lowest_rating {
+                lowest_rating = rating;
+                lowest_size = key_size;
+            }
+        }
+        lowest_size
+    }
+
     _test_roundtrip();
-    let _bytes = read("data/6.txt");
-    println!("[open] Challenge 6: {}", 0);
+    let bytes = read("data/6.txt");
+    let key_size = guess_key_size(&bytes);
+    println!("[open] Challenge 6: guessed_key_size={}", key_size);
 }